Single, simultaneous and consecutive biosorption of Cr(VI) andOrange II onto chemically modified masau stones

Albadarin, A. B., Solomon, S., Mangwandi, C., & Kurniawan, T. A. (2017). Single, simultaneous and consecutivebiosorption of Cr(VI) and Orange II onto chemically modified masau stones. Journal of EnvironmentalManagement, 204(1), 365-374. https://doi.org/10.1016/j.jenvman.2017.08.042

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Download date:03. May. 2020

1

Single, Simultaneous and Consecutive Biosorption of Anionic Cr(VI) and Orange II onto 1

Chemically Modified Masau Stones 2

1. Ahmad B. Albadarin1,2*, Samuel Solomon1, Tonni Agustiono Kurniawan3, Chirangano Mangwandi2, 3

Gavin Walker1 4

1School of Natural Sciences, Bernal Institute – University of Limerick – Limerick, Ireland. 5

2School of Chemistry and Chemical Engineering – Queen's University Belfast – Belfast BT9 5AG, Northern Ireland UK. 6

3College of Ecology & Environment, Xiamen University, Xiamen 361102, Fujian Province, China 7

*Corresponding author: E-mail: Ahmad.B.Albadarin@ul.ie. Tel: +353-(0)-61-237732. 8

ABSTRACT 9

Novel and low cost chemically modified masau stone (CMMS) was investigated for its 10

biosorption of an anionic azo dye, Orange II (OII), and toxic hexavalent chromium (Cr(VI)) from 11

aqueous systems: individually, simultaneously and consecutively. The effects of pH, contact 12

time and initial concentration (Co), and loading order on mechanisms of biosorption/reduction 13

of OII and Cr(VI) onto CMMS were examined in detail. Several analytical techniques were 14

employed to characterise the physio-chemical properties of the CMMS and determine the 15

biosorption mechanisms. The pseudo second order and redox models were able to adequately 16

predict the kinetics of biosorption. 17

The Langmuir maximum OII biosorption capacity (qmax) was calculated as 136.8mg/g for the 18

dye onto the Cr(VI)-loaded CMMS consecutive system at Co = 100mg/dm3. The qmax for the 19

Cr(VI) system was found to be 87.32mg/g at the same Co max. XPS and FTIR analyses indicated 20

the introduction of quaternary-Nitrogen to the CMMS surface after activation and the 21

involvement of carboxyl, sulphonate and hydroxyl groups in OII and Cr binding mechanisms. It 22

was confirmed that the biosorption of OII and Cr(VI) mainly takes place via two different 23

mechanisms i.e. hydrogen bonding and electrostatic attraction for the dye, and biosorption-24

coupled reduction for Cr(VI). 25

Keyword: Orange II; Hexavalent Chromium; Binary; Consecutive Biosorption; Bioremediation. 26

1. INTRODUCTION 27

Dyes and metals are pollutants found in wastewaters (Albadarin and Mangwandi, 2015; 28

Albadarin et al., 2014b). These types of environmental pollutants are often toxic, carcinogenic, 29

2

and pose serious problems, even in minute concentrations (Ferhat et al., 2016). More than 8000 30

chemical products are associated with the dyeing process with 2% of the annually produced dyes 31

(Almost 109 kg, 70% of which are azo dyes) discharged directly in aqueous effluents (Fanun, 32

2014; Liu et al., 2016; Salvi and Chattopadhyay, 2016). Amongst these azo dyes, Orange II (OII) 33

is one of the most widely used reactive dyes in the textile dyeing industries (Heibati et al., 2015). 34

Toxic metals such as chromium (Cr), lead (Pb), arsenic (As), and mercury (Hg) are 35

increasingly used in many areas for day-to-day activities (Naushad et al., 2016; Salameh et al., 36

2015). Hexavalent chromium, Cr(VI), oxyanions are found as contaminants in water, and when 37

compared to other toxic metals, Cr(VI) is relatively soluble in the aqueous phase over nearly the 38

entire pH range (Babel and Kurniawan, 2004; Li et al., 2017). Moreover, Cr(VI)–dyes complexes 39

used for dye fixation in wool dyeing are problematic compounds of wastewaters with the levels 40

of Cr(VI) in wool dying wastewaters detected in the range of 1–13 mg/dm3 (Correia et al., 1994). 41

Several studies have investigated the use of various physicochemical and biological 42

treatment methods of Cr(VI) and dyes, individually and simultaneously based on practicality and 43

industrial value (Anandkumar and Mandal, 2011; Kyzas et al., 2013; Li et al., 2016). Treatment 44

techniques include: ion exchange, chemical precipitation and electrodialysis which in many 45

cases present significant disadvantages such as high energy intensity, high reagent consumption, 46

etc. Adsorption and biosorption processes, on the other hand, present a low cost and highly 47

effective alternative in the removal of dyes and heavy metals from aqueous solutions (Gómez et 48

al., 2014; Kurniawan et al., 2006). As a result, much effort has been invested in the research of 49

low cost and efficient adsorbents, including the use of novel materials such as modified zeolites 50

(Song et al., 2015) and cerium immobilized cross-linked chitosan composites (Zhu et al., 2017). 51

However the use of raw biomaterials, both natural and by-product, for this purpose have 52

been shown to provide comparable adsorption capacities (Daneshvar et al., 2013; Karthik et al., 53

2017; Mishra et al., 2016; Šillerová et al., 2014; Wu et al., 2011). Previous studies have proven 54

3

that aminated biomasses are very efficient when employed for the removal of anionic dyes and 55

Cr(VI) oxyanions (Cao et al., 2014; Deng and Ting, 2005). 56

Therefore, this study was dedicated to: (i) use the chemically modified masau stone 57

(CMMS), for the first time, to remove Orange II (OII) and Cr(VI) from single aqueous solution; 58

(ii) compare the results from (i) in removing the two anions from multi-component systems and 59

consecutive biosorption (iii) use various analytical techniques such as XPS, FTIR and SEM to 60

comprehensively investigate the biosorption mechanisms and determine if OII and Cr(VI) 61

compete for the biosorption sites or can be removed simultaneously or one after another. 62

2. EXPERIMENTAL METHODS 63

2.1. Preparation of chemically modified masau stone, CMMS 64

Masau stone (MS) biomass, (Ziziphus mauritiana), was collected and crushed (350‒65

500µm), repeatedly washed with distilled water and dried at 90ºC until constant weight. The 66

chemical modification procedure of MS was carried out as follows: (i) 4g of the cleaned MS was 67

mixed with a solution containing 60 cm3 of 1.5M NaOH and 40 cm3 epichlorohydrin on a 68

hotplate at 40°C for 45min; (ii) the MS was then filtered and washed several times with deionized 69

water and dried; (iii) after that, the MS obtained from (ii) was mixed with 60 cm3 of 1.5M NaOH 70

and 10 cm3 of diethylenetriamine (DETA) and the mixtures was stirred at 60°C for 90min; (iv) 71

finally, the CMMS was filtered, washed with deionized water and dried in an oven at 90°C 72

overnight. 73

2.2. Hexavalent chromium (Cr(VI)) and Orange II (OII) 74

Potassium dichromate (K2Cr2O7) and Orange II sodium salt (C16H11N2NaO4S) from 75

Sigma–Aldrich (UK) were used to prepare stock solutions and subsequently diluted with 76

deionized water to required concentrations. Hexavalent chromium, Cr(VI), concentrations were 77

determined using a standard method (Albadarin et al., 2011a) using UV/Vis Perkin Elmer 78

LAMBDA 25, UK spectrophotometer at λ = 540nm. Total chromium concentrations were 79

analysed by an inductively coupled plasma (ICP-AES Perkin-Elmer 400 series) at wavelength 80

4

285nm. The trivalent chromium concentrations were calculated as: Cr(III) = CrTotal – Cr(VI) 81

and the OII dye concentrations were determined by UV/Vis spectrophotometer at λ = 478nm 82

(The UV–vis absorption spectra of this dye at different pH were identical). 83

2.3. Biosorption experiments 84

Solution pH effects (pH range 2 – 8) were investigated (volume = 25cm3; Co = 85

100mg/dm3 and temperature = 20ºC) by adding CMMS to glass jars containing the solutions 86

(Dose = 2g/dm3) and measuring the equilibrium concentrations of Cr(VI) and OII at 1 pH unit 87

increments. Samples were shaken for 24hrs on a mechanical shaker and filtered through a 88

0.45µm membrane filter to remove the CMMS. The optimal pH samples were used for surface 89

characterisations after the biosorption processes. 90

Kinetic experiments with a solution volume of 250cm3 were performed on hotplate 91

stirrers for 6hr using the same parameters and conditions. The Cr(VI)-loaded CMMS and OII-92

loaded CMMS were collected, washed with deionized water and dried. As the adsorbates were 93

chemically bonded onto the adsorbent it was assumed that no leaching occurred, which was 94

further confirmed by SEM and XPS results. Consecutive biosorption was investigated by using 95

the Cr(VI)-loaded CMMS and OII-loaded CMMS for biosorption of OII and Cr(VI), 96

respectively. The isotherm studies for the metal and dye single solutions, multi-component and 97

consecutive solutions were carried out at Co = 50mg/dm3 ‒ 300mg/dm3 and 20ºC and the 98

solutions were continuously shaken for 24h. All experiments in this section were carried out in 99

duplicate with negligible error margin. The metal and dye loading, q (mg/g), and percentage of 100

removal (%) were calculated using Eq. (1) and (2), respectively: 101

VM

CCq eo

(1) 102

The percentage removal %1001

o

e

C

C (2) 103

where Co is the initial and Ce is the equilibrium concentration of adsorbates in mg/dm3, M 104

is the amount of CMMS in g and V is the volume in dm3. 105

5

2.4. Determination of surface characteristics and functional groups 106

The surface functional groups of the CMMS were determined by Fourier Transform 107

Infrared (FTIR) Spectroscopy using a Perkin Elmer Spectrum 100 within the range of 4000–108

400/cm. The zeta potential measurements were carried out using a Malvern Zetasizer (3000HS). 109

The specific surface area of maCMMS was measured by the N2-BET method. For Scanning 110

Electron Microscope (SEM) analysis, samples were coated with gold and vacuumed (5−10min) 111

for electron reflection prior to analysis on a JEOL-JSM 6400 scanning microscope. The X-ray 112

photoelectron spectroscopy (XPS) analysis was mainly employed to verify the oxidation state of 113

the Cr bound to the CMMS surface. The Kratos ULTRA spectrometer was used for the XPS 114

measurements with the following parameters: sample temperature = 20-30oC; X-Ray Gun mono 115

Al Kα 1486.58 eV; 150 W (10mA, 15kV) and pass Energy = 160eV for survey spectra and 20eV 116

for narrow regions. 117

3. RESULTS AND DISCUSSIONS 118

3.1.Surface area, zeta potential and functional groups 119

The BET surface area of the unloaded chemically modified masau, CMMS, was measured 120

as 77.32m2/g. The total pore volume and average pore radius CMMS are 0.081cm3/g and 121

21.32Å, respectively. After Cr(VI) biosorption induced reduction, the surface area of the Cr(III) 122

loaded CMMS increased to 132.35m2/g. The zeta potential profile of CMMS showed a strong 123

pH-dependence with the CMMS adsorbent exhibiting positive zeta potential values at pHs lower 124

than 3.8 (Figure 1A), which may be due to the formation of positive CMMS‒NH3+ sites at lower 125

pH values. A pH above this value initiates deprotonation, resulting in a decrease in biosorption 126

capacities of the CMMS adsorbent, especially for anionic species (Albadarin et al., 2011b). The 127

FTIR analysis for the raw and chemically activated MS is shown in Figure 1B. The broad band 128

located at 3358–3377/cm for the MS in Figure 1B can be assigned to ‒OH, ‒SiOH and ‒NH 129

stretching vibrations of hydroxyl groups. The peak around 2900/cm is attributed to alkyl ‒C‒H 130

stretching. 131

6

Peaks around 1422/cm are due to ‒C‒H bending and asymmetric ‒SO3 bands can be 132

detected around 1329/cm (Deng et al., 2003). After chemical activation, peaks around 2100/cm, 133

1600/cm and 1100/cm are assigned to ‒C≡N, ‒NHCO and ‒C‒N stretching, respectively. This 134

demonstrates that the chemical modification has embedded many amine groups onto the CMMS 135

surface. Also, a stronger broad band around 3100 to 3700/cm indicates that several ‒OH and ‒136

NH groups were formed on the surface of the CMMS. The new hydroxyl and amine groups will 137

contribute to the removal of Cr(VI) and OII and increase the biosorption capacity via the 138

formation of columbic forces, hydrogen bonds and π‒ π interactions. The quantitative XPS 139

analysis from the high resolution spectrum indicated that the concentration of nitrogen atoms 140

nearly doubled after MS modification. This confirmed that more ‒NH groups were introduced to 141

the surface of the CMMS. More details are provided in section 3.6.2. 142

3.2. Effect of solution pH 143

The biosorption of Orange II (OII) and Cr(VI) in single and multi-component systems as 144

functions of solution pH are shown Figure 1. It was found that for single systems, above 90% of 145

OII dye and 80% Cr(VI) removal occurred at pH = 2 and 3. For the Orange II dye, the lowest 146

removal was observed at pH 8, approximately 60%, which is considered a very good removal 147

percentage bearing in mind the Co of the dye and amount of CMMS used. 148

The pH dependence of Cr(VI) and OII simultaneous uptakes were rather comparable to 149

those in their corresponding single pollutant systems as revealed in Figure 1. For the multi-150

component system, the CMMS was still able to remove substantial amounts of Cr(VI) and OII 151

at pH 2, and 4. This indicated that the anionic species of Cr(VI) and dye did not compete for the 152

biosorption sites when co-existing in the same solution and instead, OII biosorption improved 153

over the pH range studied. Similar observations were reported for the adsorption of OII and 154

Cr(VI) using quaternary ammonium salt modified chitosan magnetic composite adsorbents (Li 155

et al., 2016). However, it is worth mentioning that the possibility of Cr(VI) reduction by the 156

modified chitosan was not considered in this previous study. 157

7

The CMMS can attract the OII dye molecules by both electrostatic attraction and through 158

the formation of surface hydrogen bonds/π‒ π interactions between the amine and hydroxyl 159

groups on the CMMS surface and the nitrogen and oxygen atoms of OII dye. The good OII 160

biosorption over the entire pH range could be attributed to the interaction between the CMMS 161

surface and π-electron system of the dye. The more noticeable decrease in the biosorption 162

efficiency for Cr(VI) is typically common when using bio-based-materials where the biosorption 163

mechanism is described by the biosorption-reduction model (Albadarin et al., 2014a). At low 164

pH, Cr(VI) anions can oxidize secondary alcohol groups while being reduced to Cr(III) cations. 165

After biosorption, both Cr(VI) and Cr(III) species were found in the aqueous solution with 166

approximately 10% of the Cr(VI) that initially existing converted to Cr(III). At pH > 4, 167

dissociation of functionalities such as ‒COOH, ‒SO3 and ‒SiOH leads to increased negative 168

charge on the CMMS, thus, anionic Cr(VI) species might be repelled at these pHs. At pH = 7 169

where the minimum amount of Cr(VI) was removed (42.3% removal), only 2% of the reduced 170

Cr(VI) was detectable in the aqueous solution. The bound Cr(III) species can also form surface 171

complexes with the protonated functional groups, such as ‒NH2, ‒COOH and very active ‒SiOH 172

groups on the CMMS surface. The removal of Cr(VI) decreased by about 10% in the multi-173

component system; the equilibrium Cr(III) concentration was less than 5% of the Cr(VI) initial 174

concentration. This is attributed to the existence of Cr(III) in the solid phase, acting as a ligand 175

between the dye molecules and the CMMS adsorbent, and additionally reducing the repulsion 176

forces between the dye molecules. 177

Also, after Cr(VI) was biosorbed and reduced to Cr(III) to the CMMS, the surface area of 178

the CMMS increased as indicated by the BET results. Similar observations were published for 179

Cr(III)-loaded adsorbents used for further anionic dyes biosorption (Bouberka et al., 2006), in 180

which the BET surface area increased from 110 to 317m2/g after Cr(III) loading. This 181

demonstrates that there is an ample number of active sites which the Cr(VI) and OII molecules 182

can bind to i.e. unoccupied sites as well as to the Cr(III)-intercalated sites. However, in order to 183

8

avoid the precipitation of OII dye, all following experiments were carried out at approximately 184

pH = 3.5. 185

3.3. Contact time: single, simultaneous and consecutive biosorption 186

The contact time experiments were performed using fresh CMMS for single and multi-187

component systems and Cr(VI)-loaded-CMS and OII-loaded-CMMS for OII and Cr(VI) 188

consecutive biosorption at pH = 3.5, respectively, and the results are revealed in Figure 2. 189

Results indicate that the biosorption phenomena occurs over a short time period. The plots show 190

that the kinetics of biosorption primarily consist of two periods: an initial rapid period associated 191

with the instant external surface biosorption of metal ions/molecules. The fast removal of OII 192

and Cr(VI) is perhaps due to the electrostatic attraction, extracellular bio-reduction, micro-193

precipitation and cellular affinity (Mungasavalli et al., 2007; Volesky, 2007). 194

The second slower period was the gradual biosorption stage that occurred before metal 195

ions and dye molecules uptake reached equilibrium. The time required for OII and Cr(VI) to 196

reach equilibrium was very similar, however, the initial rate of reaction for OII seemed faster but 197

gradually decreased due to the limited number of biosorption sites at the fixed concentrations. 198

A related increase in the Cr(III) concentration (5 to 10% of Co of Cr(VI)) in the aqueous solution 199

was observed as the Cr(VI) decreased with time, (Figure 2). The slower Cr(VI) removal was 200

attributed to the fact that Cr(VI) was first biosorbed onto CMMS and then reduced to Cr(III) 201

(Albadarin et al., 2013). Cr(III) was not initially present in the solution and this confirmed that 202

Cr(VI) was reduced to Cr(III) when in contact with the CMMS surface. 203

When Cr(VI) and OII co-existed in the same solution, the simultaneous biosorption of OII 204

dye was very fast and efficient. This suggested that the energetically less favourable sites become 205

available for biosorption with an increase in the ions/molecules concentration. As for Cr(VI), the 206

removal percentage decreased but the amount of desorbed Cr(III) as a result of repulsion with 207

the positively charged functional groups on the CMMS surface decreased. These results were in 208

good agreement with the pH results. 209

9

In order to confirm these phenomena, the loaded-CMMS materials were used for Cr(VI) 210

and OII consecutive biosorption at pH = 3.5 and Co = 100mg/dm3. Interestingly, the cross-211

matched loaded-CMMS materials could efficiently biosorb OII and biosorb-reduce Cr(VI) with 212

very similar percentage removals to those obtained from the multi-component systems. This 213

confirmed that the biosorptions of OII and Cr(VI) were taking place by two different mechanisms 214

i.e. hydrogen bonding/π‒ π interactions and electrostatic attraction for the dye, and biosorption 215

coupled reduction for Cr(VI). 216

3.4. Kinetic modelling 217

The pseudo first and second order (Ho and McKay, 1999; Lagergren, 1898) and intra-218

particle diffusion models were used to describe the kinetic data tested: 219

tk

et eqq 11

(3) 220

t

tqk

qkq

e

e

t

2

2

2

1 (4) 221

where k1 (1/min) is the pseudo first order, k2 (g/mg min) is the pseudo second order rate 222

constants. 223

It has been proven that the Cr(VI) reactions which take place on the activated and raw 224

biomass surfaces do not follow simple reaction order kinetics due to the finite number of surface 225

sites available for the reaction to occur. So, the kinetics of Cr(VI) biosorption-reduction onto 226

CMMS was modelled using the redox model, as follows (Park et al., 2007): 227

ooOCredoxOC

2

ooOC

[Cr(VI)]))[Cr(VI)]B][(CB]exp(k[C

Cr(VI)][B][Cr(VI)][C] Cr(VI)[

t (5) 228

where kredox is the rate coefficient, B is the biomass and COC indicates the content of the 229

equivalent organic compound per unit gram of biomass, mg/g. 230

The fittings for the pseudo first and second order models and their calculated constants 231

are shown in Figure 2 and Table 1. It can be concluded that, to a certain extent, both models 232

were able to represent the kinetic data (R2 ≥ 0.980). This is also in agreement with previous 233

10

studies (Albadarin et al., 2012; Albadarin et al., 2014b). The fact that these models are empirical 234

equations and do not give a precise explanation of the chemical and physical processes, should 235

be considered. However, in the case of Cr(VI) biosorption onto fresh CMMS and OII biosorption 236

onto Cr(VI)-loaded CMMS, the second-order-model was able to describe the biosorption process 237

with higher accuracy (R2 value close to unity and low difference between calculated qe and qexp). 238

The k1 and k2 values for the OII biosorption onto CMMS are higher than that for the Cr(VI). 239

These values decreased for Cr(VI) biosorption when co-existing with OII and declined further 240

for the consecutive biosorption. On the other hand, k1 and k2 values increased for the OII 241

biosorption onto CMMS. Larger k values suggest that for these systems, a shorter time is needed 242

to reach a specific fractional uptake; as shown in Figure 3, the fractional uptake f vs time, t, 243

where f = qt/qe. Also, the calculated values for the rate constant of external mass transfer, ks, 244

determined from the plots of Ct/Co against time for all systems are given in Table 1 (plots are not 245

shown here). The ks increased for OII but decrease for Cr(VI) in multi-component and 246

consecutive systems. This is attributed to the OII molecules having less competition for 247

accessible surface area as the biosorption of Cr(III) increased the number of active sites. The 248

reduction reaction of Cr(VI) to Cr(III) is very fast, the fact that the biosorption of OII dye is 249

faster than that for Cr(VI), support the assumption that Cr(VI) was first electronically attracted 250

to the positively charged groups before being reduced to Cr(III) by neighbouring electron-donor 251

groups (Albadarin et al., 2013). 252

The redox model fitting for the biosorption of Cr(VI) onto fresh CMMS is also shown in 253

Figure 2. The model was able to accurately predict Cr(VI) biosorption data, confirming that 254

Cr(VI) was reduced when put in contact with CMMS. The model assumes that the rate equation 255

of Cr(VI) reduction is a first order equation with respect to both Cr(VI) concentration and 256

concentration of organic compound at constant pH and temperature. The redox reaction rate 257

constant, kredox, (Table 1) and content of organic compounds, COC, values decreased when Cr(VI) 258

11

co-existed with OII in the biosorption system. This could be due to the partial, however low, 259

competition between the Cr(VI) ions and the dye molecules before Cr(VI) reduction to Cr(III). 260

Remarkably, according to Table 1, the values of redox model constants, kredox and COC, for 261

CMMS are very similar to the second order model constants, k2 and qe (mg/g), values. This 262

indicates that the second order model was able to predict the Cr(VI) biosorption mechanism 263

(biosorption-reduction) and that the redox model accurately determined the organic compounds 264

available for this reaction to take place. These models are useful in describing the removal 265

mechanisms in spite of the lack of a complete understanding about the redox reaction between 266

Cr(VI) and various unknown components on the CMMS surface. 267

3.5. Isotherm studies: single, simultaneous and consecutive biosorption 268

The biosorption data for Cr(VI) and OII was tested by Langmuir (Langmuir, 1916), Freundlich 269

(Freundlich, 1906) and Redlich-Peterson (Redlich and Peterson, 1959) isotherm models: 270

Langmuir isotherm:

e

ee

bC

bCqq

1max

(6) 271

Freundlich isotherm: n

eFe CKq /1= (7) 272

Redlich-Peterson isotherm: βeR

eR

eCa

CKq

+1= (8) 273

where qmax (mg/g) and b (dm3/mg) are the Langmuir isotherm constants; KF (mg/g 274

(dm3/mg)1/n) is the biosorption capacity and 1/n is a measure of biosorption intensity; KR (dm3/g) 275

and aR are the Redlich-Peterson isotherm constants, where 0 ≤ β ≤ 1. 276

The isotherm constants for the biosorption processes of OII and Cr(VI) onto CMMS at 277

different systems are presented in Table 2 (Examples for the isotherm fittings are included in the 278

supplementary data, SD1). Levels of Cr(III) detected in the Cr(VI) biosorption systems were less 279

than 7% at all concentrations and, in this case, Cr(III) concentrations were not considered in the 280

isotherm modelling. From Table 2, the Redlich-Peterson isotherm model was able to characterise 281

the biosorption process in most cases with similar R2 values to those for the Langmuir isotherm. 282

12

This suggests monolayer biosorption dynamic chemisorption processes by the biosorption 283

affinity in terms of surface functional groups and bonding energy where the biosorption 284

occurring at definite localized sites that are identical and equivalent. Also, this suggests that there 285

is no steric hindrance between that Cr(VI) and the OII molecules. 286

For the Cr(VI) biosorption onto CMMS, the process was also described well by the 287

Freundlich isotherm, and the β value for the Redlich-Peterson model was the smallest, indicating 288

that the process is not restricted to the formation of monolayer. The R2 values for the Langmuir 289

and Redlich-Peterson isotherms were relatively low for the Cr(VI) binary biosorption system. 290

This behaviour could be explained as a change in the mechanism of the chromium biosorption 291

process. Chromium was initially biosorbed and reduced onto the first layer of the biosorbent 292

surface as Cr(VI) at low chromium concentration; however, when it reached its saturation, 293

another biosorption phenomenon (i.e. complexation) occurred by means of a chromium (Cr(VI) 294

and Cr(III)) biosorption process onto the multi-layer CMMS surface. The 1/n values for those 295

two systems were the lowest (closer to zero) showing that the CMMS surface is more 296

heterogeneous. KF values increased for Cr(VI) and OII binary biosorption systems confirming 297

that the CMMS has a greater biosorption tendency towards the adsorbates in these systems. So, 298

it is obvious that Cr(VI) and OII are biosorbed differently. 299

3.6. Proposed mechanisms 300

3.6.1. XPS analysis 301

3.6.1.1. Analysis for MS and CMMS before biosorption 302

The surface analysis of raw MS and CMMS was performed using XPS. The survey scans 303

of these samples indicated the presence of carbon, oxygen, nitrogen, sulphur and silicon (Table 304

3). It seems that the major component among these N moieties was already present on the MS, 305

and as indicated, corresponded to the pyrrolic-N. The already existing amine groups on the 306

surface of the biomass participate in the reaction, and the increase in concentration nitrogen 307

promotes the removal of anions (Cao et al., 2014; Deng and Ting, 2005). The N spectrum (Figure 308

13

SD2) of MS is related to one component of N: pyrrolic-N (399.8 eV). However, after activation, 309

the CMMS shows two peaks corresponding to pyrrolic-N (399.8 eV) and quaternary-N 310

(402.0eV) (Matsoso et al., 2016). 311

The C spectra of MS and CMMS are de-convoluted into four different constituents of 312

carbon at 284.8 corresponding to C–C/C=C, 286.4 related to C–O/C–N (alcohol/epoxy/alkoxy 313

and N–sp2–C (graphitic and pyrrolic) and N–sp3–C (defected sp3–C bonds), 287.98 for C=O 314

(carboxyl)/N–C=O groups, and 289.0 corresponding to O–C=O (Figure SD3) (Khandelwal and 315

Kumar, 2016). It was also found that apart from the increase in the intensity of oxygen at 532.9 316

(Spectrum not shown), no new components were detected after activation. This peak corresponds 317

to oxygen in OH groups and its concentration increased from 27.7% to 28.3% after activation. 318

The peak observed at 101.2 eV is attributed to Si2+. The decrease of the concentration for this 319

peak after biosorption indicated that Si might be involved in the reduction of Cr(VI) and 320

complexation of Cr(III). Also, the MS and CMMS contained comparatively small quantities of 321

S with narrow range spectra (~167eV) demonstrating the existence of oxygenated sulphur in the 322

form of sulphonate groups. 323

3.6.1.2. Analysis for CMMS after biosorption 324

The oxidation state of the Cr bound to the CMMS was determined using XPS and the 325

results are shown in Figure 4. The high-resolution spectra collected from (A) Cr onto fresh 326

biomass; (B) Cr onto fresh biomass followed by loading with OII; (C) Cr onto fresh biomass in 327

binary system; and (D) Cr onto OII-loaded biomass. For Cr2O3, major bands appeared at binding 328

energies of 577.0–578.0eV, corresponding to Cr2p3/2 orbital, and 586.0–588.0 eV matching 329

Cr2p1/2 orbital. On the other hand, the CrO3 is characterized by higher binding energies; 581.0–330

582eV and 590.0–591.0eV, as the hexavalent form draws electrons more strongly than trivalent 331

form. 332

From Figure 4, by relating the Cr peaks detected, it is possible to determine whether the 333

bound Cr is in trivalent or hexavalent form. It can be confirmed that Cr(VI) was mostly reduced 334

14

to Cr(III) when brought into contact with the CMMS biosorbent, however, some Cr(VI) can still 335

be detected. This confirms that the removal mechanism of Cr(VI) by CMMS involves three steps 336

i) attraction of Cr(VI) ions to positively charged groups such as ‒CN+; ii) reduction of Cr(VI) to 337

Cr(III) by neighbouring electron-donor groups such as ‒SiOH; iii) Cr(III) ions are bio-338

sequestrated through metal ion coordination, ion exchange and chelating activities and then, part 339

of the reduced Cr(III) ions are freed into the aqueous solution because of electronic repulsion 340

(Albadarin et al., 2011b; Albadarin et al., 2014a). These conclusions are in good agreement with 341

the previously reported findings for the biosorption of Cr(VI) using seaweed (Murphy et al., 342

2009). An estimation of the Cr(III)/Cr(VI) ratios in the different biosorption systems show that 343

the CMMS is more efficient to bio-reduce hexavalent chromium than other biosorbents i.e. 344

seaweed (Yang and Chen, 2008). 345

The elemental compositions of the Cr(VI)-loaded CMMS in the different biosorption 346

systems are summarised in Table 3. It is obvious that the CMMS had relatively large amounts of 347

Cr linked with their surface after reaction. The slight changes found in the atomic concentration 348

of carbon indicate that some CMMS leached during the experiment. The change in the 349

concentration of oxygen may be due to the biosorption of chromium hydroxides (Murphy et al., 350

2009). The Cl element is coming from the activation reagent and the increase in the S an 351

indication of OII biosorption. From Table 3, the amount of loaded Cr before or after loading the 352

OII had comparable amounts of chromium bound to the CMMS surface (0.47 and 0.52%, 353

respectively) whereas a lower amount was bound to the CMMS in the binary system (0.27%) 354

and a maximum quantity was loaded in the Cr(VI) single system. These results agree with 355

previous sorption isotherm results presented in Table 2. Finally, the shifts in the peak at 400.0eV 356

indicate that hydrogen bonding/π‒ π interactions and electrostatic attraction through –N=N– is 357

contributing in the removal mechanisms of OII dye. 358

15

3.6.2. FTIR and SEM analysis 359

FTIR spectroscopy was used in section 3.1. to identify the functionalities capable of 360

interacting with Cr(VI) and OII. The same technique is used in this section to determine the main 361

functionalities involved in the biosorption process. The spectra for the Cr(VI) and OII-loaded 362

CMMS in single and binary systems were recorded (data not shown here). The shifts and shape-363

changes taking place in the –OH stretching band around 3430/cm indicates that the Orange II is 364

attached to the oxygen atoms on the CMMS forming monodentate, bidentate or tridentate bonds 365

and substituting the water molecules (Albadarin and Mangwandi, 2015; Benjamin and Leckie, 366

1981). The decrease in the intensity for the peak around 2100/cm indicate that ‒C≡N group was 367

involved in the biosorption process. Also the shifts of the peaks around 1600 to 1625/cm and 368

disappearance of the sulphonate group around 1329/cm after Cr(VI) and OII biosorption confirm 369

the involvement of the ‒C=O and ‒SO3 in the process. New individual peaks at 1520 and 1600 370

1/cm due to benzene skeleton vibrations appear in the FTIR spectrum of OII-loaded CMMS. 371

Based on the analysis from the XPS, FTIR and the SEM, the removal of Cr(VI) and Orange II 372

from aqueous solutions can be illustrated as shown in Figure 5. It can be seen that electrostatic 373

attraction, hydrogen bonding and π‒ π interactions mechanisms are involved in the removal 374

process. 375

Finally, the Scanning electron micrographs showing the MS (A), CMMS (B), Cr-loaded 376

CMMS (C) and Orange II-loaded CMMS (D) morphology are presented in Figure 6. From the 377

images the fresh surfaces of the MS surface appear rough with a rugged morphology. After 378

chemical activation, the CMMS looks darker and more heterogeneous, making it a potential for 379

the biosorption of Orange II dye and Cr(VI) ions. The images present significant differences in 380

surface morphology between the MS and CMMS; these surface topographies variations are due 381

to the different quantities of available functionalities i.e. ‒NH2 and ‒OH. After OII and Cr(VI) 382

biosorption, the morphology of the CMMS changed and the edges of the microstructure appear 383

to be less obvious than before the chromate ion biosorption. This confirms the loading of Orange 384

16

II and Cr(VI) onto the CMMS and indicate that ion exchange might be involved in the 385

biosorption process, especially after Cr(VI) reduction. 386

4. CONCLUSIONS 387

It can be concluded from the findings of this study that CMMS is capable of simultaneously 388

and consecutively removing both Cr(VI) and OII from synthetic wastewater by biosorption. The 389

CMMS biosorbents has preferential biosorption of OII over Cr(VI) in their aqueous mixtures at 390

acidic conditions, which is due to the fact that CMMC shows higher affinity to dye than to metal 391

ions. The performance of the CMMS on removal of OII was better, in consecutive and binary 392

systems. This suggests that the presences of Cr(IV) and OII ions in the systems does not result 393

in competition for biosorption sites but enhance the removal of the ions. It is hypothesised that 394

the improvement in the removal of OII biosorption is due to reduction in the electrostatic 395

repulsive forces between the large dye to presence of the chromium (III) ions. The above 396

conclusions and the high capacity for OII and Cr(VI) make this chemically modified by-product 397

biosorbent suitable for real wastewater treatment applications. 398

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506

507

508

509

510

511

512

513

514

(A)

(B)

19

515

Figure 1: Zeta potential (A) and FTIR (B) analyses of CMMS biosorbent and the effect of pH 516

on the biosorption of Orange II dye and Cr(VI) in single (left) and binary (right) systems. 517

518

20

519 Figure 2: The fittings for the pseudo first, second order and redox models for the 520

biosorption of OII dye and Cr(VI) onto CMMS. 521

21

522

Figure 3: The OII and Cr (VI) fractional uptakes, f, vs time in different systems. 523

524

525

526

527

Contact time (min)

0 50 100 150 200 250 300

f (q

t/q

e)

0.0

0.2

0.4

0.6

0.8

1.0

Cr(VI) onto Orange II-Loaded CMMS

Cr(VI) onto fresh CMMS/binary

Cr(VI) onto fresh CMMS

Orange II onto Cr(VI)-Loaded CMMS

Orange II onto fresh CMMS/binary

Orange II onto fresh CMMS

22

528

Figure 4: High resolution Cr2p spectra for CMMS biosorbent. 529

530

531

Binding energy (eV) Binding energy (eV)

23

532

Figure 5: Schematic diagram illustrating the mechanisms of OII and Cr(VI) removal by 533

CMMS biosorbent in different systems. 534

535

536

24

537

Figure 6: Scanning electron micrographs showing the MS (A), CMMS (B), Cr-loaded 538

CMMS (C) and OII-loaded CMMS (D) surface morphologies. 539

540

541

1 Supplementary Data 542

543

A B

C D

25

544

545

Ce (mg/L)

0 10 20 30 40 50 60

qe (

mg

/g)

20

40

60

80

100

120

OII onto Cr-loaded CAMS

Langmuir isotherm

Freundlich isotherm

Redlich-Peterson isotherm

Ce (mg/L)

0 20 40 60 80 100 120 140

qe (

mg

/g)

20

40

60

80

Cr(VI) onto fresh CAMS/Binary

Langmuir isotherm

Freundlich isotherm

Redlich-Peterson isotherm

26

SD1: Isotherm fittings for the biosorption of OII onto Cr(VI)-loaded CMMS and Cr(VI) biosorption onto unloaded CMMS. 546

547

SD2 : The XPS spectrum for N in MS and CMMS. 548

549

399.8 eV 400.0 eV

401.9 eV

27

550

551

SD3: Carbon spectrum for raw Masau Stone (MS) and chemically activated Masau Stone (CMMS). 552

28

553

554

555

Table 1: Kinetic constants determined for the biosorption of OII and Cr(VI) onto CMMS 556

in different systems. 557

System Pseudo first order Pseudo second order External mass

transfer*

qexp qe k1 R2 qe k2 R2 ks

Cr(VI) only 43.44 40.11 0.027 0.980 44.93 9.0 × 104 0.995 0.061

Cr(VI)/binary 38.44 37.33 0.019 0.994 44.85 5.0 × 104 0.993 0.038

Cr(VI) onto Orange

II-CMMS

38.52 39.43 0.014 0.997 40.20 3.0 × 104 0.993 0.018

Orange II only 49.01 47.63 0.062 0.997 51.55 1.8 × 103 0.993 0.095

Orange II/binary 49.18 47.76 0.080 0.988 50.30 2.4 × 103 0.994 0.111

Orange II onto

Cr(VI)-CMMS

49.56 47.96 0.145 0.990 50.79 3.9 × 103 0.982 0.153

Redox model

kredox COC R2

Cr(VI) only 4.0 × 104 44.03 0.984

Cr(VI)/binary 2.0 × 104 41.80 0.998

Cr(VI) onto Orange II-

CMMS

1.0 × 104 40.44 0.992

*Rate constant of external mass transfer determined from the plots of Ct/Co against time. 558

559

560

561

562

29

Table 2: Isotherm constants for the biosorption processes of OII and Cr(VI) onto CMMS at different systems. 563

System Langmuir isotherm Freundlich isotherm Redlich-Peterson isotherm

qmax b R2 KF 1/n R2 KR aR β R2

Cr(VI) only 87.32 0.066 0.978 17.57 0.320 0.988 11.21 0.314 0.819 0.997

Cr(VI)/binary 66.99 0.082 0.923 18.45 0.249 0.847 5.208 0.069 0.998 0.923

Cr(VI) onto OII-

CMMS

81.26 0.048 0.974 15.80 0.336 0.883 3.407 0.015 0.987 0.985

OII only 116.5 0.111 0.995 26.82 0.325 0.943 14.32 0.149 0.955 0.995

OII/binary 129.2 0.165 0.967 32.47 0.335 0.925 24.26 0.239 0.938 0.969

OII onto Cr(VI)-

CMMS

136.8 0.125 0.942 31.39 0.341 0.845 13.75 0.049 0.997 0.952

564

30

565

566

567

568

569

570

571

572

Table 3: Elemental composition of unloaded and OII and Cr(VI)-loaded CMMS as 573

determined using XPS. 574

Elements MS CMMS Cr onto

CMMS

Cr onto CMMS

followed by loading

with OII

Cr onto CMMS in

binary system

Cr onto OII-

loaded CMMS

O 1s 27.70% 28.30% 30.55% 33.01% 32.22% 30.66%

C 1s 68.90% 67.32% 64.54% 62.13% 63.28% 65.89%

N 1s 2.10% 3.20% 3.26% 3.25% 3.17% 2.00%

Cl 2p ‒ 0.20% 0.25% 0.12% 0.25% 0.23%

S 2p 0.30% 0.28% 0.26% 0.37% 0.32% 0.34%

Si 2p 1.10% 0.70% 0.44% 0.65% 0.49% 0.06%

Cr 2p ‒ ‒ 0.70% 0.47% 0.27% 0.52%

Cr(VI)/Cr(III) ‒ ‒ 23.07% 23.68% 32.42% 21.73%

575

576